CINCINNATI — A Silicon Valley startup developing electric propulsion systems for satellites has raised $10 million and added the billionaire founder of LinkedIn to its board.Apollo Fusion announced the $10 million Series B round July 11, bringing the total the company has raised to date to more than $18 million. The round was led by venture fund Greylock Partners, with one of the fund’s partners, Reid Hoffman, joining the board.Apollo Fusion will use the funding to scale up manufacturing and testing facilities for its electric propulsion system, called the Apollo Constellation Engine (ACE). The company believes that ACE is well-suited to serve the growing demand for smallsats with onboard propulsion.Several other companies, including Accion Systems, Enpulsion and Phase Four, are also working on a variety of electric propulsion systems intended for use on smallsats. Apollo Fusion argues, though, that ACE offers more performance in a smaller package.“We believe that one of the most important things to measure systems on is the total impulse per unit mass of the propulsion system and total impulse per unit volume of the propulsion system,” said Mike Cassidy, chief executive of Apollo Fusion, in an interview. “For all the published stats ...